Heart failure is the leading cause of morbidity and mortality world-wide. Current heart failure treatments are not effective in enhancing myocardial repair or regenerating lost heart muscle. Improving myocardial repair after myocardial infarction will require enhancing vascular supply to regions with marginal perfusion and stimulating myocardial regeneration through formation of new cardiomyocytes and supporting vasculature. The epicardium, a polarized epithelium covering the heart, is an essential regulator of fetal myocardial growth and coronary vasculogenesis. Epicardium and myocardium engage in elaborate paracrine signaling to regulate each other's development. Furthermore, epicardial cells undergo epithelial to mesenchymal transition (EMT), generating epicardium-derived mesenchymal cells (EPDCs) that migrate into the heart and differentiate into fibroblasts, vascular smooth muscle cells, endothelial cells, and potentially cardiomyocytes. In the adult heart, epicardium is an important modulator of the myocardial injury response, and recent studies indicate that the developmental properties of epicardium may be harnessed for therapeutic regeneration. Our preliminary data show that chemically modified mRNA (m*RNA) drives transient, high level paracrine factor expression in the heart. VEGF-A m*RNA, delivered once at the time of experimental myocardial infarction (MI), enhanced capillary density, reduced infarct size, improved ejection fraction, and enhanced survival for up to one year. Epicardial progenitors, marked by expression of the transcription factor Wt1 (Wilms Tumor Suppressor 1), were a major target of VEGF-A m*RNA activity. VEGF-A expanded and mobilized post-MI WT1+ epicardial progenitors. Remarkably, VEGF-A m*RNA altered the fate of these cells, enhancing their differentiation into endothelial cells and cardiomyocytes and reducing their differentiation into myofibroblasts. Our data suggest a novel therapeutic paradigm, in which brief activation of paracrine signaling pathways alters resident progenitor cell fate to achieve sustained therapeutic benefit. This cell-free therapeutic paradigm can be readily translated to large animal and clinical studies. In this proposal, we further investigate the regulation of adul epicardial cell behavior develop the therapeutic paradigm advanced by our preliminary data, through the following Specific Aims: (1) Determine the mechanism by which VEGF-A redirects EPDC fate. (2) Define the role of Wt1 in regulating adult epicardial progenitor activity in the normal and injured adult heart. (3) Identify additional factors with beneficial activity in myocardal infarction. By combining novel m*RNA technology with the Pu lab's established expertise in epicardial progenitors and their role in fetal and adult heart, this proposal will lead to mechanistic insights into myocardial regeneration, advance application of m*RNA technology to myocardial regeneration, and lead to new avenues for clinical translation.